Pacemakers for generating artificial stimulating pulses for the heart, and which may be implanted in the body, are well known. Originally the electrical circuitry for such pacemakers was of analog design, but in recent years digital circuitry has been also employed. A digital approach to pacemakers has led to the evolution of programmable pacemakers--pacemakers having parameters such as pulse rates which are adjustable (programmable) once the pacemaker has been implanted. The programs can be changed from outside the patient's body by appropriate signal transmission to the implanted pacemaker and without surgery. Programmable pacemakers are described in, for instance, British Patent Specifications Nos. 1,385,954 and 1,398,875. Such pacemakers have circuitry to detect and decode signals transmitted outside the body and alter the program accordingly. In British Patent Specificaton No. 1,385,954 the programming is accomplished by means of a magnetic field which is sensed by a magnetic reed switch; the opening and closing of the switch providing programming pulses to a program store. In British Patent Specification No. 1,398,875 the programming is by means of radio frequency transmission and reception.
Many pacemakers are of the demand type--that is they only supply a stimulating pulse to the heart when a natural heart beat is absent. To accomplish this, demand pacemakers have means for sensing the presence or absence of natural heart beats and for actuating the stimulating pulse as appropriate.
Demand pacemakers normally have an input amplifier which receives the electrical signals detected at the heart, and the amplified signals are then employed to control the demand function of the pacemaker. Generally, signals provided by the input amplifier are employed to inhibit any artificial pulse being generated by the pacemaker: the absence of such signals being taken by the pacemaker as indicating the absence of a normal heart beat and causing artificial stimulating pulses to be generated.
Pacemakers are implanted in the body for a period of years, and during that time various parameters can change causing the effective signals supplied by the input amplifier to change in magnitude. Typical of such parameters is an increase in impedance of the body tissue as "seen" by the pacing electrode. But most prevalent is the wide range of electrode placement effectiveness which can result in not only very large but very small signals.